An active-matrix substrate for use in a liquid crystal display device and other devices includes switching elements such as thin-film transistors (which will be hereinafter referred to as “TFTs”), each of which is provided for an associated one of pixels. As such switching elements, a TFT which uses an amorphous silicon film as its active layer (and will be hereinafter referred to as an “amorphous silicon TFT”) and a TFT which uses a polysilicon film as its active layer (and will be hereinafter referred to as a “polysilicon TFT”) have been used extensively. Recently, people have proposed that an oxide semiconductor be used as a material for the active layer of a TFT instead of amorphous silicon or polysilicon. Such a TFT will be hereinafter referred to as an “oxide semiconductor TFT”. Since an oxide semiconductor has higher mobility than amorphous silicon, the oxide semiconductor TFT can operate at higher speeds than an amorphous silicon TFT. Also, such an oxide semiconductor film can be formed by a simpler process than a polysilicon film, and therefore, is applicable to even a device that needs to cover a large area.
On an active-matrix substrate for use in a display device, a TFT is provided for each pixel and functions as a switching element. Each TFT has its drain electrode connected to a pixel electrode. For example, Patent Document No. 1 discloses an active-matrix substrate for use in an IPS (in-plain switching) liquid crystal display device. In the active-matrix substrate disclosed in Patent Document No. 1, a pixel electrode and a TFT's drain electrode are connected together inside a contact hole cut through an interlayer insulating layer.